A fixing device, a drying device, an erasing device, or a printing device of an electrophotographic image forming apparatus has a roller-shape or belt-shape fixing member and a roller-shape or belt-shape pressure members. The fixing member and the pressure member nip and carry a recording material having an unfixed image or a printed image, constituted of developer such as toner, which is formed thereon. The developer is melted and dried by heating so that the developer image is fixed onto the recording material.
For example, as shown in FIG. 3 describing the present embodiment, an electrophotographic image forming apparatus includes: a photoreceptive drum 22; an image forming section having various means provided around the drum; and a fixing section (fixing device 23) having a fixing device provided in downstream with respect to the image forming section in the flow of a recording material. In the image forming section, a toner image is formed on the photoreceptive drum 22 and transcribed onto the recording material. In the fixing section, the recording material is put into the nip section provided between a fixing member (fixing roller 231) and a pressure member (pressure roller 232) so that the toner image is heated and melted to be fixed onto the recording material while the recording material is being carried.
In the nip section, the fixing member and the pressure member are charged by (i) friction between the fixing member and the pressure device, and (ii) friction between the recording material and the fixing member, and (iii) friction between the recording material and the pressure member. An electrostatic effect which results from the charging causes a so-called offset phenomenon in which the toner adheres to the fixing member. When a large amount of toner adheres to the fixing member due to the electrostatic offset, this raises the following problems: after the fixing member has rotated 360°, the toner adheres again from the fixing member to the recording material to taint the output image side; or after the toner has accumulated as toner taint on a temperature sensor and a releasing nail, the toner adheres to the recording material under some conditions to taint the recording material.
Conventionally, in order to prevent the above electrostatic offset, as described in Japanese Patent Publication No. 2734146/1991 (Tokkyo 2734146; registered on Jan. 9, 1998) (Prior Art 1) for example, a bias voltage having the same polarity as the toner is applied to a fixing member to make the toner electrostatically repulsive so that toner taint caused by the electrostatic offset may be reduced. In addition, as described in Japanese Unexamined Patent Publication No. 305580/1999 (Tokukaihei 11-305580; published on Nov. 5, 1999) (Prior Art 2), a pressure member is made conductive and is grounded so as to restrain a fixing member and the pressure member from being charged so that toner taint caused by the electrostatic offset may be reduced.
However, as in Prior Art 1, when the bias voltage is applied to the fixing member, it is necessary that the pressure member is constituted of an insulating member or a high-resistance in order to prevent the bias voltage from leaking. Fluorine plastics are generally used to form a surface covering layer of the pressure member. A nonconductive fluorine plastic tube is negatively charged in terms of electrical properties. As shown in FIG. 16, the higher the fixing speed is, the higher the electrical potential is. For example, in the case of a high-speed machine whose printing speed is 50 sheets per minute or more (the fixing speed is 250 mm/s or higher), the electrical potential reaches as high as −3 to −5 kV. Meanwhile, a bias voltage of only up to about −2 kV can be applied to the fixing device due to the low pressure-resistant capacity of the coating layer (usually of fluorine plastics) of the fixing device. As a result, the Coulomb force toward the fixing member acts on the toner (usually negatively charged) on a recording material so that, as shown in FIG. 16, there still occurs electrostatic offset.
Moreover, for example, as is often the case with the contact point to the releasing nail, when the surface of the pressure member is partially damaged, impedance (surface resistance) of that part changes. As a result, the bias voltage applied to the fixing member undesirably leaks along that part, so that electrostatic offset corresponding to flaws on the surface of the pressure member occurs in a streak manner.
Meanwhile, as in Prior Art 2, when a pressure member is made conductive and is grounded, the pressure member is not be charged. This is effective in reducing the electrostatic offset of toner on the surface of a recording material. However, the toner adhering to the back of the recording material offsets to the pressure member. As a result, electricity removing means in contact with the pressure member becomes tainted over time with toner, paper dust, and the like. This lowers an electricity removing effect, so that the electrostatic offset occurs.